Although the atrial natriuretic polypeptide (ANP) hormone secreted by atria plays a major role in the medically important function of fluid, electrolyte, and blood pressure homeostasis, the cellular mechanisms by which stretching atria stimulates hormone secretion are unknown. The long-term goal of this proposal is to investigate the mechanisms of stretch-secretion coupling in atria and its interaction with atrial contraction by studying in detail the Ca2+-sensitive mechanotransducer relationship between stretch-activated atrial ion transport and ANP secretion demonstrable in intact rat atria. The relationship will be examined (a) by in vitro studies on intact atria stretched by a physiological range of distending pressures in which stretch- activated cellular net Na efflux, Ca influx, depolarization, and ANP secretion can be measured together during variations of passive stretch and contraction frequency while conditions determining Ca2+ influx across the plasma membrane or Ca2+ release from sarcoplasmic reticulum (SR) can be separately varied; (b) by single channel patch-clamp studies on cultured and freshly dissociated cardiac myocytes to identify and characterize stretch-activated channels and/or second messenger systems; (c) by applying fura-2 fluorescence microscopy interfaced with digital computer-aided image analysis to single atrial myocytes (both cultured myocytes and freshly dissociated myocytes) to investigate the time course and subcellular localization of the changes in cytoplasmic Ca2+ associated with changes in ANP secretion due to variations in ionic environment, membrane stretch, contraction rate, activation or inhibition of second messenger systems, and other variables; (d) by correlating stretch-activated ANP secretion with stretch-activated changes in tension and in plasmalemmal Na-Ca exchange using measurements of tension and intracellular Na+ activity in quiescent and contracting rat atrial trabeculae; and (e) by correlating ANP secretion rates of atrial myocytes cultured from adult rats with observations of microtubule-associated cytoplasmic translocation of atrial granules from the Golgi to the plasma membrane using electron microscopy and double immunofluorescence microscopy while stretch, Ca influx, and SR Ca release can be separately varied and second messenger systems can be stimulated or blocked in different ways.